1. Field of the Invention
This invention relates to systems for dispensing foam polymer material, and more particularly to systems which maintain the polymer material under pressure with foaming gas in solution and which dispense the polymer/gas solution to form the foam.
2. Description of the Prior Art
The assignee of the present invention has pioneered the development and application of methods and apparatus for foaming and dispensing polymeric materials such as hot melt thermoplastic adhesives, polymeric coatings, paints and other thermoplastic and/or thermosetting materials.
This technology has been used in the application of hot melt adhesives in which it has been found that the adhesive strength of a bond achieved with a given volume of selected hot melt adhesive can be appreciably improved, and in most instances at least doubled, if the adhesive were applied as a foam rather than as a conventional non-foamed adhesive. Foam adhesive systems encompassed by this technology are commercially available from the assignee of the present invention under the trademark FoamMelt.RTM..
A hot melt thermoplastic adhesive foam system is disclosed in U.S. Pat. No. 4,059,466 wherein a solid mixture of hot melt thermoplastic adhesive and blowing agent is heated and melted in a heated reservoir at a temperature above the melting temperature of the adhesive but below the decomposition temperature of the blowing agent. The molten adhesive and solid blowing agent mixture is then pressurized by a gear pump and supplied under pressure as, for example, 300 pounds per square inch (psi) to a hot melt dispenser. Between the pump and the outlet of the hot melt dispenser, the molten adhesive and solid blowing agent mixture is further heated to a higher temperature at which the blowing agent decomposes and evolves as a gas as for example, nitrogen, which at that pressure goes into solution with the liquid adhesive. The pressurized liquid/gas adhesive solution is then supplied to a valved-type outlet at the adhesive dispenser from which the adhesive is dispensed at atmospheric pressure. Upon emerging from the outlet nozzle of the dispenser, the gas evolves from the solution in the form of small bubbles causing the adhesive to expand volummetrically. The resultant adhesive in an uncompressed state sets up as a homogeneous solid foam having gas cells substantially evenly distributed throughout the adhesive.
In U.S. Pat. No. 4,059,714, another hot melt thermoplastic adhesive foam system is disclosed in which the molten adhesive is mixed with a gas and pressurized by either a one-step or two-step gear pump. Within the gear pump, the gas and molten adhesive are thoroughly mixed and the gas is forced under pump outlet pressure into solution with the liquid adhesive. The pressurized liquid/gas adhesive solution is then supplied to a valved-type dispensing gun from which the adhesive is dispensed at atmospheric pressure. Again, upon emerging from the outlet nozzle of the dispenser, the gas evolves from the solution in the form of small bubbles causing the adhesive to expand volummetrically and forming in an uncompressed state a homogeneous solid foam having gas cells evenly distributed throughout the adhesive.
This technology has been very successful in producing foamed adhesives. The insulating effect of the gas bubbles adds to the open time of the adhesive, and reduces its working viscosity. Thus the adhesive spreads easier and covers more surface area, reducing consumption of the adhesive. Other advantages are increased bond strength, longer open times for product positioning, faster set times, stronger bonding to porous or irregular surfaces, improved bonding to conductive materials, reduced adhesive consumption, increased production rates, lowered labor costs and better product appearance.
The extension of this technology to other polymeric materials, such as thermoset sealant materials presented certain problems. Whereas hot melt adhesives have a viscosity typically in the range of about 2,200 cps to 20,000-35,000 cps, "high" viscosity polymeric material such as thermoset materials used as adhesives, seals and gasketing material have viscosities in the range of about 50,000 cps to about 1,000,000 cps. The low viscosity foaming gas is more difficult to mix successfully with the highly viscous sealant material so as to achieve a uniform solution without creating undesirable heat and other problems. Many of these difficulties were solved by using a bulk mixer and associated apparatus as described in U.S. Pat. No. 4,778,631, to Cobbs, Jr., et al., wherein a low energy input disc mixer is employed to force the low viscosity foaming gas into solution with the "high" viscosity polymeric material. The mixer may be driven by a constant speed motor, which is monitored by a torque sensor. The mixing apparatus may also require a bulk melter for the material that is fed to the mixer, a cooling system with a supply of cooling fluid, and a pressurized supply of foaming gas including a pump. The apparatus uniformly blends the foaming gas with curable sealant materials to produce high-performance gaskets. When the solution is released to atmosphere, a homogeneous foam is formed wherein the gas is released from solution and becomes entrapped in the polymer.
Systems for producing foams of such "high" viscosity materials are commercially available from the assignee of the present invention under the trademark FoamMix.RTM.. The systems can produce sealants that are foamed in place, creating closed-cell foam seals that act as effective, long-lasting barriers against air, dust, vapor and fluids in various applications. The sealant may be any pumpable material, such as polyurethane, silicone or plastisol. This technology produces a foam without any chemical reaction, without any chemical blowing agent and without any volatiles. Since chemical reactions are not used in the foaming process, the chemical composition of the sealant material is not changed. Foamed sealants retain their basic physical properties such as temperature and chemical resistance. The gas content of the foamed material is typically 50% by volume, but the amount of gas in the solution can be adjusted to control material durometer, compression set resistance and flexibility. The use of this "foam-in-place" technology reduces the use of expensive materials such as polyurethanes and silicones and provides improved compressibility, improved resilience and reduced cure time. The technology is advantageously used to produce foam sealant which can be applied by robotic devices, replacing the old, labor-intensive manual method of applying die-cut gaskets. Automated foam-in-place gasketing increases production, reduces labor and material costs, and improves quality through accurate and consistent gasket placement.
Whether dispensing "high" viscosity foam materials or lower viscosity foam adhesives, it is very important to maintain proper pressure on the polymer/gas solution through the dispensing process. Wide pressure fluctuations can result in premature formation of the foam within the dispenser if the pressure of the solution falls below the critical pressure level required to maintain the gas in solution. If the foam forms prematurely, the foam can shear as it leaves the dispenser, creating a foam layer with an uneven texture which not only is presents a rough appearance but also reduces the effectiveness of the foam. Conversely, if the pressure on the polymer/gas solution is too high within the dispenser, the gas will not readily leave solution when the material is dispensed. These problems increase when the foamed polymeric materials are dispensed intermittently in a production environment. When the polymeric material is dead-ended or stopped within the dispenser, unsatisfactory variations in the amount of material discharged from the dispenser can occur, and each opening and closing of the valves associated with the dispenser to obtain intermittent discharge of material can result in pressure fluctuations.
Many of the problems associated with intermittent application of high viscosity polymeric materials have been addressed in U.S. Pat. No. 5,207,352, to Porter et al. which discloses an apparatus for dispensing a solution of highly viscous polymeric material and a gas which comprises a dispenser, a pressure regulator and a swivel mount, all of which are interconnected to one another. The pressure regulator is adapted to be connected to a source of a pressurized polymer/gas solution either directly or through the swivel mount. The solution is transmitted through the pressure regulator directly into a fluid passageway formed in the dispenser body of the dispenser. Minimal pressure drop due to line losses occurs because of the close proximity of the pressure regulator and dispenser, and the solution is maintained under high pressure within the fluid passageway in the dispenser to the discharge outlet of a nozzle carried by the dispenser. This configuration maintains the gas in solution in the polymeric material within the dispenser body until it is discharged from the nozzle to atmosphere to form a homogeneous foam having gas cells substantially evenly distributed through the polymeric material.
As indicated in U.S. Pat. No. 5,207,352, it is very important to maintain the proper pressure on the polymer/gas solution throughout the dispenser until the material emerges from the dispenser to atmospheric pressure and the gas can evolve from the solution to form the foam. These problems are magnified if the material is dispensed in a wide band instead of a small bead. It would be very desirable to apply hot foam adhesives in a wide band in the manufacture of various products. For example, in the manufacture of disposable diapers or training pants as shown in U.S. Pat. No. 5,246,433, a hot melt adhesive is applied to a flap member using a melt blown application system. However, heretofore it has not been possible to dispense a pressurized polymer/gas solution to provide a wide band of foam material. The addition of another dimension to the dispensing profile greatly complicates the task of maintaining the proper pressure on the polymer/gas solution within the dispenser.
In order to dispense the solution in a wide band, a slot-type dispenser must be used, and many inherent problems arise in attempting to dispense solution through a slot dispenser. The material must be uniformly distributed across the width of the slot. In addition, as the material is distributed across the width of the slot, the pressure must be maintained on the solution to prevent premature foaming of the material. If the material begins to foam before it has left the dispenser, the foam shears as it leaves the dispenser, and as it is applied to the substrate. This premature shearing of the finished foam produces a layer of material having a unacceptably rough texture and reduces the effectiveness of the foam. In addition, the flowrate of the material leaving the dispenser must match the rate at which the substrate passes beneath the dispenser. The configuration of the slot and of the distribution manifold leading to the slot must be capable of being used with different materials having different viscosities and at various flowrates, so that the dispenser can be used over a range of production rates.